1. Field of the Invention
The present invention relates to a metal vapor discharge lamp for mainly performing near-infrared radiation, a projector equipped with the lamp, and a lighting device for metal vapor discharge lamps.
2. Description of the Related Art
For security during the night, recording of comings and goings of visitors and/or monitoring/detection of intruders is now being performed.
Such monitoring or detection is realized by photographing subjects or objects using, for example, a CCD camera. To photograph them, near-infrared radiation, which cannot be sensed by human beings, is widely utilized, since if visible light is used, the existence of a camera can be known, and the light may be dazzling and make them feel uncomfortable.
If an electric bulb is used as the light source, much near-infrared light is irradiated but the efficiency of visible light is lower than discharge lamps. Further, Jpn. Pat. Appln. KOKOKU Publication No. 4-70736, for example, discloses a near-infrared illuminator and photographing device for security that use a low-pressure rare gas discharge lamp for emitting a large amount of near-infrared light, the kind of rare gas in the lamp and its sealed pressure being suitably set.
The low-pressure rare-gas discharge lamp of this publication employs a relatively long glass bulb. Therefore, when this lamp is used as a near-infrared illuminator or in a near-infrared photographing device, if it is fixed in a certain position indoors or outdoors, no problems will occur. However, if the lamp is used in a mobile unit, such as a vehicle, it disadvantageously requires a large space. Further, if the lamp uses coiled thin filaments as electrodes, the filaments may be easily broken since they do not show a high resistance against impact or vibration. This means that stable light emission cannot be expected for a long time.
The inventors of the present application have recently paid attention to a metal vapor discharge lamp used as a headlamp for, for example, vehicles. This lamp emits a large amount of near-infrared light and can be made as small as an electric bulb.
More specifically, halogen lamps are generally used as headlamps for vehicles in order to enhance the forward visibility for safety during the night or in tunnels. However, high-pressure metal vapor discharge lamps, such as metal halide lamps of a short arc type, have recently been employed, which emit light of a higher intensity and larger quantity than halogen lamps and have electrodes of a high resistance against impact or vibration.
Traffic accidents during the night between vehicles and pedestrians are not decreased, although those between vehicles are decreasing. Therefore, there is a demand for a means that enables drivers to quickly see pedestrians and avoid accidents. Metal vapor discharge lamps can illuminate a larger area than conventional halogen lamps since the former lamps emit higher intensity light, thereby enabling drivers to have a higher visibility and show a quicker response.
However, illuminating a larger area brightly means that pedestrians may be dazzled and feel uncomfortable even at a distance.
In light of this, there is a need for a high-pressure metal vapor discharge lamp used as, for example, the headlamp of a vehicle or a monitor lamp in a banking facility, which can irradiate pedestrians or customers with near-infrared light and visible light of moderate intensity.